sub what_is
{
     my ($file) = @_;
     my $header;

     open (FILE, $file) || return;
     read (FILE, $header, 12);     # Read first twelve bytes
     close (FILE);

     return unless (length($header) == 12);

     return "GIF"  if (substr($header,0,3) eq "GIF");
     return "JPEG" if (substr($header,6,4) eq "JFIF");
     return "PNG"  if (substr($header,1,3) eq "PNG");
     return;
}